chapter 6 form 5 variation

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Azam Sensei 4

Variation in Human Wilt Chamberlain, a

famous NBA basketball player, and Willie Shoemaker, a famous horse racing jockey.  Chamberlain was 7 feet, 1 inch tall, and weighed 275 pounds. Willie Shoemaker was barely 100 pounds and only 4 feet, 11 inches tall.

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Definition

• The differences in traits or characteristics between individuals / organisms of the same speciesTwo types of variation :

1. Discontinuous variation2. Continuous variation

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Discontinuous Variation• An organism either has the

characteristic or it doesn`t have it.

• There is no range of these characteristics between extremes.

• An organism can easily be placed into definite categories

• There is no disagreement about the categories

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…Discontinuous Variation

• These characteristics are usually qualitative – they cannot be measured

• They are the result of genes only

• They are not affected by the environment

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Distribution of blood group types among individuals..

There can be no dispute about these categories. In the ABO blood group system there can only be A, B, AB or O

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Examples of discontinuous variation…

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Continuous Variation• Every organism within one species

shows the characteristic, but to a different extent.

• The characteristic can have any value within a range

• These characteristic are usually quantitative, they can be measured

• They results from both genes and the environment

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Human height is an example of continuous variation.

There are many possible intermediate classes between the two extremes.

NORMAL DISTRIBUTION

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Causes of variation

• Environments factors

• Genetics factors

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Environmental Factors

• Sunlight

• Temperature

• Nutrition

• Soil condition (edaphic factors)

• Water

• Humidity

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The effect of on humans

• As a person is exposed to more sunlight , more melanin is produced which makes skin darker

• Two identical twins have the same genotype, one twin may be heavier if he eats more food than the other twin who has less to eat.

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Soil pH - Hydrangea

• Acidic – blue flower

• Alkaline – Pink flower

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Temperature – Primula sinensis

• Warm – white flower

• Cool – red flower

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Genes and environment both produce variation

• Pea plants are either tall or short because of the genes they inherit.

• However, all the tall pea plants are not exactly the same height and neither are all the short pea plants exactly the same height.

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Several environment factors can influence their height :

• They may not all receive the same amount of light and so some will not photosynthesis as well as other.

• They may not all receive the same amount of water and mineral ions from soil : this could affect the manufacture of a range of substances in the plant

• They may not all receive the same amount of carbon dioxide, less photosynthesis.

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Genetic Factors

• Crossing – over

• Independent assortment

• Mutation

• Random fertilisation

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CROSSING OVER during prophase 1

• An exchange of portions of chromatids between homologous chromosomes

• Resulting in new, different genetic combinations of genes from the parents. The new genetic combinations result in variations.

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INDEPENDENT ASSORTMENT during meiosis

• During metaphase 1 , homologous chromosomes arrange themselves randomly at the equator plate.

• The random arrangement and separation of each homologous pair is independent of one another.

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INDEPENDENT ASSORTMENT

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INDEPENDENT ASSORTMENT

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MUTATION

Definition

• A sudden permanent change in the nucleotide sequence of the DNA or in the amount of the DNA

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Mutation can arise because of :

mistake in copying of DNA as cells get ready to divide – pairing with the `incorrect` base.

Damage to the DNA – some environmental factor might alter the bases present in the DNA

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Uneven distribution of chromosomes during the division of cells

• mutation in somatic cannot be inherited but affects the person during lifetime.

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Mutagen = agents that cause mutation

• High-energy ioning radiation : X-rays, ultraviolet rays, gamma rays. The radiation penetrates the nucleus of a cell and causes changes which can damage the structure of genes, chromosome or the DNA molecule.

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…mutagen

Chemical mutagens :

• Benzene, formaldehyde, asbestos, carbon tetrachloride, mustard gas, tar in tobacco, pesticides

• Bind to DNA and separates the two stands

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2 two types of mutation : Gene & chromosome

Gene mutation ( = point mutation)• Occur when part of the DNA on a

single chromosome is change.• A change in the sequence of bases

may result in a defective protein or no protein being produced at all.

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As a result , can lead to a considerable change in a characteristic

E.g : sickle cell anaemia, albinism,polydactylism

33polydactylism

Sickle-cell anaemia

albino

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Types of gene mutationType of gene mutation Description

Base Duplication

CAGTTGCATCG

CAGTTGGCATCG

A base of nucleotide is repeated

Base Insertion

CAGTTGCATCG

CAGTTGCTATCG

A base of nucleotide is inserted into the chain

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Types of gene mutationType of gene mutation Description

Base Deletion

CAGTTGCATCG

CAGTTG_ATCG

A base of the nucleotide chain is removed from the sequence

Base substitution

CAGTTGCATCG

CAGTTGCGTCG

A base substituted with other base .

Eg : Base A substituted with base G

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Chromosomes mutation

Involves changes in :

• The structure of chromosomes

• The number of chromosomes

There is either an addition or loss of chromosomes in the gametes.

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Increase or decrease in chromosomes number caused by :

•The failure of homologous chromosomes to segregate

•During meiosis 1 and meiosis 11

•(text book page 199) ; figure 6.6

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Types of chromosomal mutation

Deletion• As the name implies,

genes of a chromosome are permanently lost as they become unattached to the centromere and are lost forever

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Types of chromosomal mutation

Duplication 1 Normal chromosome

before mutation 2 Genes from the

homologous chromosome are copied and inserted into the genetic sequence

3 New chromosome possesses all its initial genes plus a duplicated one, which is usually harmless

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Chromosome contains inverse gene sequence

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Chromosomal translocation

Non- homologous chromosomes

Chromosome contains translocated genes

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Increase or decrease in chromosomes number caused by :

•The failure of homologous chromosomes to segregate

•During meiosis 1 and meiosis 11

•(text book page 199) ; figure 6.6

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Draw figure 6.6 , text book page 199

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Normal gametes : nNormal somatic cells : 2n

• Several set of chromosomes : polyploidy

• Three sets : triploid (3n)• Four sets : tetraploid (4n)• Common among flowering plant and

food plant• Produce bigger fruit with more

nutritional value.

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Chromosomes mutation caused :

Genetic disease :

• Down`s syndrome ( male and female)

• Turner`s syndrome (female)

• Klinefelter`s syndrome (male)

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Down`s syndrome : 47, extra choromosome 21

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Turner`s syndrome : 44 + XO

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Women with Turner syndrome have :

• ovaries that don't develop properly, resulting in infertility.

• health problems and obvious physical symptoms, such as short stature and a webbed neck, but rarely the condition goes undetected until a woman experiences infertility and has chromosome analysis.

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Klinefelter`s syndrome : 44 +XXY, extra X choromosome

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About 1 in 1000 men are born with XXY chromosomes.

• Men with XXY can appear quite normal or can develop different symptoms of varying severity, such as small testes, enlarged breasts, lack of body hair, and mild retardation. In rare cases, a man will have more extra copies of the X chromosome, making him XXXY or even XXXXY.

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Most men with Klinefelter syndrome lack sperm..

• but there are cases where XXY men have fathered children.

• In addition, researchers have been able to find sperm in the epididymis of men with Klinefelter syndrome, opening the door to assisted reproduction using intracytoplasmic sperm injection (ICSI) in vitro fertilization.

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The importance of variation in the survival of a species

• Adaptations to compete and to survive in new environments that arise.

• Prevent extinction of species

• Eg camouflage ( prevent predation)

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• SPM 2003

• Question 3

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Answer SPM 2003 Q3

3(a) (i) Student X : loops Student Y : Composite

(ii) genetic factor (iii) 1. random combinations of chromosomes

during meiosis in gametogenesis 2. crossing over / recombinations

3. random fertilization combination of different chromosomes in

gametes in sexual reproduction.

Any one

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(b) (i) discontinuous variation

(ii) ability to roll the tongue /

ear lobe attached / blood

group / kind of hair / dimple

/ eye colour

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(c) P1 : a simple evidence of either the fingerprints or a photograph only is not enough to identify an individual // several evidence / characteristics of an individual are necessary for one`s identification. P2 : the fingerprint of an individual is permanent / does not change with time.P3 : the photograph of an individual can easily be altered.

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(d) 1. body mass shows continuous variation while fingerprints show discontinuous.

2. body mass is not distinctive while the graph for fingerprint is distinctive

3. graph for body mass shows a normal distribution while the graph for the fingerprint shows a discrete distribution.